Conventional voltage regulators that have a configuration in which, to stabilize the output voltage, a feedback voltage that is proportional to the output voltage is compared to a reference voltage using an operational amplifier. The conductive state of an output transistor is controlled according to the results of that comparison. This configuration is useful when the input voltage is generally higher than the intended output voltage. But a voltage regulator may also be required to control output voltage even when the power supply voltage is lower than the operating range, such as at the time of power activation. For this reason, a configuration is used in which the power source voltage is monitored by a power source voltage-monitoring circuit, and the voltage regulator is started up when the power source voltage has risen to a sufficient level for the voltage regulator to operate.
However, in a method where a voltage regulator is started up only when the power source voltage becomes sufficient, the time (“start-up” time) from power activation to when the voltage regulator begins to operate becomes relatively long. There is also the problem that, by including a power source voltage-monitoring circuit with the voltage regulator circuit, the size of the semiconductor device that includes the voltage regulator circuit increases.